1. Field of the Invention
The present invention relates to an optical information medium comprising one disc formed of a transparent substrate and having an optical recording layer and a reflecting layer at least on one surface thereof and another disc which is bonded to one disc by an adhesive.
2. Prior Art
A digital video disc (DVD) capable of recording and reproducing information with high-density has been now put into practical use associated with the recent development and practical utilization of laser light having a short wavelength. The optical information medium of this type can adopt a structure for bonding two pieces of discs.
A read-only optical information medium having such a structure for bonding two surfaces of discs has a center hole at the center thereof and a clamping area at the outside thereof. A pair of discs which are bonded to each other include at least one transparent substrate comprising at one surface thereof, an information recording area formed outside the clamping area, pits serving as an information recording means formed on the information recording area, and a reflecting layer formed of a metal film and provided on the pit.
Further, as an optical information medium capable of recording information, a tracking guide is formed on the information recording area of at least one substrate, and the substrate is coated with organic dye, etc. by means of a spin-coating process, to form an optical recording layer, and a reflecting layer formed of a metal film is formed on the optical recording layer. Still further, the entire surfaces of a pair of discs are coated with a reactive curing resin as an adhesive by means of a spin-coating process or a screen printing, and the coated surfaces are laid on top of the other to oppose each other, and two discs are bonded to each other by the adhesive set forth above.
In a WORM (write once, read many times) optical information medium, there is employed a system for coating the transparent substrate with a dye material such as an organic dye with a given thickness to form an optical recording layer, and forming a reflecting film such as a metal film on the optical recording layer.
In such a WORM optical information medium, if the optical recording layer of the disc is coated with an adhesive such as a reactive curing resin by a spin coating process or screen printing, etc., and two discs are bonded to each other by this adhesive, residual stress is generated in the joining boundary between the different materials, whereby the discs are liable to warp and the adhesive surface is liable to flake due to the warpage of the discs. Further, there occurs a problem that when a shock is applied to the discs, the discs are liable to flake, thereby deteriorating the shock resistance. Particularly, the flaking is liable to occur at the edges of the two discs, which causes a problem of brittleness against the shock on the edges. Still further, since the adhesion between the two discs deteriorate, the moisture in the air is liable to enter the discs, which causes a problem of deterioration of the layer with moisture.
If a reaction-sensitive resin is employed as an adhesive layer, the shrinkage involved in the curing occurs, and the stress generated thereby influences the recording layer, thereby distorting the recording layer. As a result, a variation of tracking signal and/or reflectance occurs when the discs are reproduced with a laser beam. The variation rate exceeds 30%, thereby obstructing the record and reproduction of the information.
When the discs are bonded by the adhesive, one surface of one disc is coated with an adhesive, and another surface of another disc is overlaid on one disc, thereafter they are irradiated with light so as to cure the adhesive. When one disc is are laid on top of the other, bubbles are liable to enter between the two discs, and hence the stress is liable to occur by bubbles at the time of the bonding of the discs, and the magnitude of the stress increases. Accordingly, the influence of the stress upon the recording layer also increases, and there is a possibility that the optical disc is deformed to the extent to be recognized visually. A vacuum degassing apparatus is employed to prevent bubbles from entering the adhesive layer when the discs are bonded to each other, which makes the apparatus large-scaled, and hence increases the operating process, which causes a problem of deterioration of productivity.
It is a first object of the invention to reduce a stress even if the stress occurs owing to bubbles which are produced when two discs are bonded by an adhesive. It is a second object of the invention to reduce the shrinkage of the adhesive involved in the curing of the adhesive layer. It is a third object of the invention to restrain the variation rate of reflectance and the variation of a push-pull signal which are caused when the adhesive layer is cured, thereby permitting the signal level not to reach a critical value. It is a fourth object of the invention not to deform the discs from to the shrinkage involved in the curing of the adhesive layer. It is a fifth object of the invention to select a suitable material from an ordinary material for use in the adhesive layer, thereby dispensing with the use of a specially prepared material. It is a sixth object of the invention to perform a bonding operation using the adhesive in a simple manner without using a large-scale apparatus such as a degassing apparatus, thereby enhancing productivity. It is a seventh object of the invention to increase the resistivity of friction of the adhesive for bonding the discs.
The optical information medium comprises a pair of discs 1 and 5 which are to be bonded to each other, an optical recording layer 12 which is formed on at least one of the discs 1 and 5 at the surface where the discs 1 and 5 are bonded and a reflecting layer 13 formed over the optical recording layer 12. The optical information medium according to the present invention has an adhesive 11 for bonding a pair of discs 1 and 5 by way of the optical recording layer 12 and reflecting layer 13 in the aforementioned optical information medium, and the thickness of the adhesive 11 ranges from 10 to 80 xcexcm. A protecting layer 14 may be formed over the reflecting layer 13, and the pair of discs 1 and 5 are bonded to each other by way of the protecting layer 14.
The optical recording layer 12, the reflecting layer 13 and the protecting layer 14 to be formed on the disc 1 are sequentially directly formed in this order, but another layer may be inserted between these layers. The reactive curing resin, or a hot melt material, or the like is used as the adhesive 11, particularly, it is preferable that the shrinkage rate of the adhesive 11 be less than 15% when it cured by irradiation of UV ray.
Since the thickness of the adhesive 11 ranges 10 to 80 xcexcm, the coating layer of the UV curing resin adhesive, for instance has a sufficient viscosity, and even if bubbles are produced when the main surface the of other disc contacts the coating layer, the viscous adhesive of the coating layer flows to fill up the bubble holes. Even if the viscous adhesive of the coating layer is cured by the irradiation of the UV rays while bubbles remain therein, the stress generated based on these bubbles can be reduced by the deformation caused by the plasticity and elasticity of the resin, thereby reducing the influence upon the recording layer. If the thickness of the adhesive 11 is less than 10 xcexcm, the bubbles can not be filled up by the deformation of the adhesive 11. On the other hand, if the thickness of the adhesive 11 exceeds 80 xcexcm, the curing speed of the adhesive 11 reduces so that the adhesive 11 is liable to be non-curable.
As mentioned above, the optical information medium of the present invention can reduce the stress even if the stress is generated from the bubbles produced in the adhesive layer. Further, the shrinkage of the adhesive 11 when it cures is small, the variation rate of the reflection and the variation of push-pull signal do not reach critical level from the shrinkage of the adhesive 11 when it cures. The deformation of the adhesive when it cures is not recognized from the external appearance, and further, the adhesive can be selected from an ordinary material without using the specially prepared material, and a large-scaled degassing apparatus is not required, so that the bonding of the discs by the adhesive can be performed in the ordinary bonding manner.
Grooves 6 and 6 are defined in at least one of the discs 1 and 5 at the outer periphery of the adhesive surface thereof, and the outer peripheries of the discs 1 and 5 may be bonded to each other by the adhesive 11 filled in these grooves 6 and 6. In such an optical information medium, the adhesive 11 is filled in the grooves 6 and 6 to increase the bonding surface area by the adhesive 11 compared with a case where only the flat main surfaces of the discs 1 and 5 alone are bonded to each other by the adhesive 11, so that the resistivity of friction at the outer peripheries of the discs 1 and 5 increases by a so-called anchor effect.
The grooves 6 and 6 may be defined in the outer peripheries of the discs 1 and 5 or may be defined in the center side from the outer peripheries of the discs 1 and 5. In the latter case, since the grooves 6 and 6 do not appear on the outer peripheral surfaces of the discs 1 and 5, they present the same outer peripheral surfaces in external appearance as an optical information medium having no grooves 6 and 6.
A part 11a of the adhesive 11 bonding the discs 1 and 5 may be fixed to an outer peripheral surfaces of at least one of the discs 1 and 5. Even in such a optical information medium, the fixing surface area of the adhesive 11 increases because the adhesive 11 is fixed to the outer peripheral surfaces of the discs 1 and 5 compared with the case where the adhesive 11 is fixed to only the flat main surfaces of the discs 1 and 5, the resistivity of friction at the outer peripheries of the discs 1 and 5 increases by a so-called anchor effect.
In this case, inclined surfaces 15 and 15 which are obtuse or acute relative to the bonding surfaces of the discs 1 and 5 may be formed on the outer peripheral surfaces of the discs 1 and 5 to which the part 11a of the adhesive 11 is fixed. If the inclined surfaces 15 and 15 which are obtuse relative to the bonding surfaces of the discs 1 and 5 are formed, the adhesive 11 does not necessary to protrude largely from the outer peripheral edges of the discs 1 and 5 so that adhesive 11 does not largely influence the outer peripheral shape of the optical information medium. On the other hand, if the inclined surfaces 15 and 15 are acute relative to the bonding surfaces of the discs 1 and 5, the outer periphery of the adhesive 11 moves around to the back side of the bonding surfaces of the discs 1 and 5 and it is bonded to the back side, enhancing the anchor effect to obtain high resistivity of friction.
The discs 1 and 5 are bonded together by the adhesive 11 not only at an information recording area r on the bonding surfaces of the discs 1 and 5 but also at an inner information non-recording area i formed at the inner peripheries of the discs 1 and 5. That is, the entire surfaces of the discs 1 and 5 are bonded together by the adhesive 11 to extend from the center holes 4 to the outer most peripheries thereof. As a result, the entire surfaces of the two discs 1 and 5, including the information non-recording area i formed at the inner peripheries of the discs 1 and 5, are bonded to each other so that the resistivity of friction of the discs 1 and 5 increases and the adhesion between the discs 1 and 5 is enhanced. Particularly, since the adhesive 11 reaches the center holes 4 of the discs 1 and 5 and the edges of the center holes 4 of the discs 1 and 5 are bonded by the adhesive 11, the discs 1 and 5 hardly flake at the inner peripheries of the center holes 4, thereby enhancing the hermeticity between the discs 1 and 5.
A groove 7 is defined between the center holes 4 of the discs 1 and 5 and the information non-recording area i at the bonding surfaces, and the adhesive 11 is filled in the groove 7, thereby increasing the fixing surface area of the adhesive 11, and hence the resistivity of friction increases at the inner peripheries of the discs 1 and 5 by a so-called anchor effect. If a rough surface 8 is formed between the center holes 4 of at least one of the discs 1 and 5 and the information non-recording area i at the bonding surfaces thereof, the fixing surface area of the adhesive 11 increases by the rough surface 8 so that the resistivity of friction at the inner peripheries of the discs 1 and 5 increases. Such a rough surface 8 may be character information which is engraved on the surfaces of the discs 1 and 5, for example, a lot number or a product number.
The discs 1 and 5 are bonded in principal at their entire surfaces which oppose each other. In this case, it is preferable that the surface including an area having no optical recording layer 12 is bonded to another disc 5 without forming the optical recording layer 12 on at least a part of the surface except the information recording area r of the disc 1. In other words, a marginal part where no optical recording layer 12 is formed at the outer periphery and/or inner periphery of the information recording area r in which the signal of the disc 1 is recorded, and the surface including this marginal part where the optical recording layer 12 of the disc 1 is provided may be bonded to another disc 5. The width of the marginal part of the disc 1 is more than 0.1 mm. It is more preferable that the width of the marginal part exceed 1.5 mm.
In such an optical information medium, the optical recording layer 12 is not formed on the outside of the information recording area r of the disc 1, namely, on the marginal part outside the area whole information recording, and the marginal part is bonded directly by the adhesive 11 without interposing the optical recording layer 12. Accordingly, the discs 1 and 5 are bonded to each other with a high resistivity of friction at the marginal part. Since the marginal part locates outside the information recording area r, the resistivity of friction adjacent to the edges of the discs 1 and 5 is enhanced, thereby protecting the information recording area r inside the edges.
Meanwhile, it is possible to form a non-bonding area m at the peripheries of the center holes 4 of the discs 1 and 5 where the part of the adhesive 11 is not bonded without bonding the entire surfaces of the discs 1 and 5. In such an optical information medium, the non-bonding area m is formed on the peripheries of the center holes 4 and the adhesive 11 is not bonded onto the non-bonding area m so that the adhesive 11 is not bonded to the peripheral surfaces of the center holes 4. When the discs 1 and 5 are coated with the adhesive 11, the adhesive 11 neither passes through the center holes 4 nor moves around to the recording surfaces opposite to the bonding surfaces of the discs 1 and 5.
In such an optical information medium, the discs 1 and 5 are generally coated with the adhesive 11 and bonded thereby at the outer side thereof by a spin coating process. In this case, if the grooves 7 and 7 are defined in at least one of the bonding surfaces of the discs 1 and 5 at the area outside the non-bonding area m, when the discs 1 and 5 are coated with the adhesive 11, the adhesive 11 can stay in the grooves 7 and 7. As a result, it is possible to surely prevent the adhesive 11 from bonding to the non-bonding area m.
Meanwhile, if a center hole 4xe2x80x2 of the disc 5 is greater than the diameter of the center hole 4 of the disc 1, and it is also greater than the diameter of the non-bonding area m, the adhesive 11 does not enter the non-bonding area m when the discs 1 and 5 are coated with the adhesive 11 at the bonding surfaces thereof and they are laid on top of the other. Accordingly, it is also surely possible to prevent the adhesive 11 from bonding to the non-bonding area m. In this case, if the part of the adhesive 11 moves around and bonds to the peripheral surface of the center hole 4xe2x80x2 of the disc 5 having a large diameter, the resistivity of friction is reinforced at this portion.
The boundary of the non-bonding area m is not always circular and is disposed at a fixed range from the centers of the discs 1 and 5 but may be indefinite. That is, the boundary of the non-bonding area m may be eccentric relative to the centers of the discs 1 and 5 or the boundary of the non-bonding area m may be of a different shape. In such a manner, the maximum amplitude Q of a mechanical resonance frequency fo may be reduced, so that the oscillation of the optical information medium can be reduced at the high speed revolution thereof.